Wrought dispersion strengthened metals by powder metallurgy

ABSTRACT

THIS INVENTION RELATES TO THE POWDER METALLURGY OF WROUGHT, DISPERSION STRENGHTENED METALS AND ALSO TO A POWDER METALLURGY METHOD FOR PRODUCING WROUGHT DISPERSION STRENGTHNED METAL SHAPES SELECTED FROM THE GROUP CONSISTING OF NICKEL, COPPER, LOWER ALLOY STEELS, MARAGING STEELS, ZINC-BASE METALS, COLUMBIUM-BASE, TANTALUM-BASE AND TUNGSTEN-BASE REFRACTORY METALS, PLATINUMBASE METALS, AND GOLD-BASE METALS CHARACTERIZED METALLOGRAPHICALLY BY A UNIFORM DISTRIBUTION OF DISPERSOIDS IN BOTH THE LONGITUDINAL AND TRANSVERSE DIRECTIONS.

June 12, 1973 WROUGHT DISPERSION STRENGTHENED METALS BY POWDER METALLURGY Original Filed Aug. 27, 1969 I II [I J. S. BENJAMIN 5 Sheets-Sheet 1 .3 COOL/7N7 June 12,1973 3 BENJAMIN 3,738,817

WROUGHT DISPERSION STRENGTHENED METALS BY POWDER METALLURGY June 12, 1973. J. 5. BENJAMIN 3,733,317

WROUGHT DISPERSION STRENGTHENED METALS BY POWDER METALLURGY Original Filed Aug. 27, 1969 5 Sheets-Sheet 5 1 1973 J. 5. BENJAMIN I 3,733,817

WROUGHT DISPERSION STRENGTHENED METALS BY POWDER METALLURGY Original Filed Aug. 27, 1969 5 Sheets-Sheet 4 Ti E.

June 12, 1973 J. 5. BENJAMIN WROUGHT DISPERSION STRENGTHENED METALS BY POWDER METALLURGY Original Filed Aug. 2'7, 1969 United States Patent 3,738,817 WROUGHT DISPERSION STRENGTHENED METALS BY POWDER METALLURGY John Stanwood Benjamin, Suifern, N.Y., assignor to The International Nickel Company, Inc., New York, N.Y. Application Aug. 27, 1969, Ser. No. 853,413, now abandoned, which is a continuation-impart of application Ser. No. 709,700, Mar. 1, 1968, now Patent No. 3,591,362. Divided and this application Mar.'5, 1971, Ser. No. 121,551

US. Cl. 29-1825 Int. Cl. Bzzr 3/00 Claims ABSTRACT OF THE DISCLOSURE The present application is a division of my copending US. application Ser. No. 853,413 filed Aug. 27, 1969, now abandoned which, in turn, is a continuation-in-part of US. application Ser. No. 709,700, filed Mar. 1, 1968, now US. Patent 3,591,362.

RELATED APPLICATION In the aforementioned related application, Ser. No. 709,700, which is incorporated herein by reference, a method is disclosed for producing a wrought composite metal powder comprised of a plurality of constituents mechanically alloyed together, at least one of which is a metal capable of being compressi-vely deformed such that substantially each of the particles is characterized metallographically by an internal structure comprised of the starting constituents intimately united together and identifiably mutually interdispersed. One embodiment of a method for producing the composite powder resides in providing a dry charge of attritive element and a powder mass comprising a plurality of constituents, at least one of which is a metal which is capable of being compressively deformed. The charge is subjected to agitation milling under high energy conditions in which a substantial portion or cross section of the charge is maintained kinetically in a highly activated state of relative motion and the milling continued to produce wrought composite metal powder particles of substantially the same composition as the starting mixture characterized metallographically by an internal structure in which the constituents are identifiably and substantially mutually interdispersed within substantially each of the particles. The internal uniformity of the particles is dependent on the milling time employed. By using suitable milling times, the interparticle spacing of the constituents within the particles can'be made very small so that when the particles are heated to an elevated difiusion temperature, interdiflusion of diffusible constituents making up the matrix of the particle is effected quite rapidly.

Tests have indicated that the foregoing method enables the production of metal systems in which insoluble nonmetallics, such as refractory oxides, carbides, nitrides, silicides, and the like, can be uniformly dispersed throughout a metal particle, such as nickel. In addition, it is possible to interdisperse alloying ingredients within a refractory metal particle, such as chromium, which has a propensity to oxidize easily due to its rather-high free 3,738,817 Patented June 12, 1973 energy of formation of the metal oxide. In this connection,

mechanically alloyed powder particles can be produced STATE OF THE PRIOR ART The trend in recent years has been towards improving J the physical properties of existing commercial metals.

Examples of such metals are nickel, copper, low alloy steels, zinc-base metals, the chromium-base, columbiumbase, tantalum-base, molybdenum-base and tungsten-base refractory metals, and even the gold-base and platinumbase precious metals. The recently developed high strength maraging steels are also of interest. It would be desirable to provide such steels with improved resistance to creep at elevated temperature.

Copper, as an engineering material, has two important uses: 1) as an electrical conductor because of its very high electrical conductivity, and (2) as a structural element in heat exchangers, because of its high thermal conductivity. When alloying ingredients are added to copper to improve its strength properties, including resistance to creep, usually the electrical and thermal conductivity properties are greatly adversely afiected.

For example, the addition of 1.5% silicon to substantially pure copper as a solid solution strengthener markedly reduces the thermal conductivity by about and also reduces the electrical conductivity as referred to standard copper by about 88%, while increasing yield strength from about 8,000 p.s.i. to 15,000 p.s.i (for a l'f round). Similarly, an addition of 3% Si also greatly reduced electrical conductivity by 93% and thermal conductivity by about 90% while increasing yield strength in the annealed condition for a one inch round to about 22,000 p.s.i. Adding 5% aluminum to copper likewise adversely affects the conductivity properties by reducing electrical conductivity by about 82.5% and thermal conductivity by about 80%, while increasing yield strength in the annealed condition to about 20,000 to 25,000 p.s.i.

Another disadvantage of these copper alloys is their lack of high temperature stability at temperatures up to the melting point, whereby their strength properties are adversely affected after prolonged heating.

Recently, attempts have been made to utilize the techniques of powder metallurgy for improving the physical properties of metals employing the phenomenon referred to as dispersion strengthening. The Possibilities of this system came to light with the development of the A1-Al O system, otherwise referred to as SAP (sintered aluminum powder) investigated by Irmann and others. This system was a natural combination due to the inherent propensity of aluminum to have a thin film of alumina on its surface. In a material with a high surface-to-volume ratio,

such as a powder, the surface oxide can constitute a considerable fraction of the powder. Thus, it was found that when aluminum powder was consolidated by not pressing and extrusion, the thin oxide film ruptured, which permitted the welding together of the aluminum particles. The wrought structure produced in this way was characterized by a dispersion of alumina flakes in the alumi-v num matrix. This technique has not been generally applicable to other common metals as most other metals do not form a stable high tempera ure oxide coating of the character of alumina. One method which has been employed involves mechanical mixing which implies mixing together metal powder of the desired matrix and dispersoid powders of the desired character and size. Many practical problems are encountered, including the tendency of the refractory or dispersoid particles to flocculate due tostatic electrical surface charges and the fact that refractory particle spacing can be no closer than the diameter of the metal powders employed. Interparticle spacing is important in achieving the desired strength. Flocculation of dispersoid particles, however, is disadvantageous in that it may lead to stringersin the final product. In order to achieve closer refractory particle spacing, it is logical to go to finer metal powders. However, metal powders in the submicron size range have the attendant disadvantage of being pyrophorically active and thus prone to contamination, thereby tending to adversely affect the final wrought product. Moreover, mechanically mixed powders of constituents of different densities present the problem of segregation during storage and/or handling so that a great deal of care has to be exercised to assure a uniform mix at all times, since powder segregation is known to lead to the formation of stringers in the wrought product. i

Another method proposed is the internal oxidat on process. In this process, alloy metal powders containing a reactive solute element such as aluminum, titanium, zirconium or thorium, in dilute solid solution therew th are exposed to an elevated temperature in an atmosphere which is selectively oxidizing to the solute element. oxygen diffusion into the powder converts the reactive solute metal into fine refractory oxide particles dispersedthrough the metal matrix. As very fine metal particles are generally preferred for this purpose, contamination again is a problem. Also, it is diflicult to employ the process on matrix metals which tend to oxidize. I

The ignition surface coating process involves mixing metal or alloy powders with a liquid solution of a decomposable compound of the intended refractory oxide dispersoid to coat the metal particles with a film. For example, nickel powder can be mixed with an alcohol solution of thorium nitrate after which the mixture is dried and pulverized. The mixture is then heated in an inert reducing atmosphere to convert the salt to the corresponding oxide. Again, the need for fine metal powders in order to achieve close dispersoid particle spacing introduces the factor of contamination. Furthermore, a liquid solution tends to cause segregation during preliminary preparation since the last of the liquid to evaporate tends to be very rich in the salt it is desired to uniformly disperse. Microstructures of Wrought metal products produced by this method tend to show stringers of dispersed oxide. In addition, when using a process like the foregoing, precautions must be taken when decomposing the salt so that the coated metal particles do not pyrophorically burn up during salt decomposition. I

Another aqueous process is the mixed hydrate method in which hydrates of reducible matrix metals and the dispersoid are co-precipitate'd and the matrix metal hydrate selectively reduced in preferenc to the dispersoid, e.g., thoria hydrate. However, this process tends to be messy and, moreover, segregation can occur while handling the materials during the wet stage. Thus, stringers may be a problem.

Stringers and associated defects are deleterious to structural elements subject to loading, particularly at high temperatures. For the purpose of describing the attributes of the invention, stringers are defined as a non-uniform concentration of dispersions characterized by a longitudinal pattern in which a plurality of dispersoids appear to be agglomerated or highly concentrated or confined in a long narrow region, with regions adjacent the stringers exceeding 1 micron in minimum dimension which are impoverished in the dispersoid. Such impoverished regions do not contribute significantly to the strength'of the material and a body in which such areas constitute more than by volume will be significantly weaker than a body free of such defects. Stringers are not too apparent when a portion of a structural element or shape is viewed in transverse section, the constituents thereof appearing as dots. However, stringers are easily discerned metale iw byfixaminiflg a Wrought metal product in longitudinal section,

Stringers are not easily avoided by the powder metallurgy methods of the prior art. Although many attempts have been made to produce high quality dispersion strengthened metal structures, none, as far as I am aware, has been wholly successful prior to the present invention.

It is thus an'object of this invention to provide a powder metallurgy method for producing a wrought, dispersion strengthened, metal product in which the formation of stringers of dispersoids in the final wrought product is greatly inhibited. i

. A further object is to provide a powder metallurgy method for producing a wrought, dispersion strengthened metal product in which contamination during the early stages of manufacture is substantially inhibited due to thenature of the starting material employed.

Still another object-is toprovide a powder metallurgy method for producing a wrought, dispersion strengthened, metalproduct characterized by a uniform distribution of dispersoids in substantially any selected area of said product of average diameter ranging up to about 500 microns in size determined in both the longitudinal and transverse section.

The invention also provides as an object a powder metallurgy produced wrought, dispersion strengthened, metal product characterized by a high degree of dispersion uniformity in both longitudinal and transverse section in any selected area of average diameter of up to 500 microns, while being substantially free from stringers.

These and other objects will more clearly appear when taken in conjunction with the following description and the accompanying'drawing, wherein:

FIG. 1 depicts schematically a ball charge in a kinetic state of random collision;

FIG. 2 is a schematic representation of an attritor of the stirred ball mill type capable of providing agitation milling to produce composite metal particles in accordance with the invention;

FIG. 3 is'illustrative of dry milling curves showing the variation in hardness of composite metal particles of Ni-ThOg as a function of milling time for two types of dry milling devices;

FIG. 4 is a reproduction of a photomicrograph taken at 7 50 diameters depicting the microstructure of a wrought nickel-thoria composite metal particle produced in accordance with the invention;

FIG. 5 is a reproduction of a photomicrograph taken at 750 diameters'illustrating the structure of a composite metal particle taken from the same batch of material depisted in FIG. 4 after the powder was annealed in argon for 16 hours at 1200 C.;

FIG. 6 is a reproduction of a photograph taken at 750 diameters showing the structure of a wrought product produced by not extruding the powder shown in FIG. 4;

FIG. 7 is a reproduction of a photomicrograph taken at 750 diameters of hot extruded dispersion-strengthened nickel alloy produced in the same manner as the material of FIG. 6 except that the composite metal powder employed was milled for a longer time; and

FIG. 8 is a reproduction of a photomicrograph taken at 750 diameters showing the structure of the alloy depicted in FIG. 7 after cold swaging to provide a reduction in area of 75%.

STATEMENT OF THE INVENTION In its bi'oad aspects, the present invention is directed to the powder metallurgy production of a wrought, dispersion strengthened metal product selected from the group consisting of nickel, copper, low alloy steels, marging steels, zinc-base metals, the columbiuin-base, tantalumbase, molybdenum-base, and tungsten-base refractory metals, platinum-base metals and gold-base metals characterized by a high degree of dispersion uniformity in both the longitudinal and transverse cross sections and, a particularly, in any selected area of average diameter of p o about 500 microns or higher at a magnification of up to 10,000 times or higher. Thus, a selected area in the wrought product of average diameter of about 25 microns when magnified 10,000 diameters would show a high degree of dispersion uniformity. Such uniformity results from the use of a dense, wrought, metal composite particle having a highly uniform internal structure. In other words, by starting with the foregoing composite particles as the building blocks in producing the wrought metal shape, the high degree of uniformity of each of the composite particles is carried forward and maintained in the final wrought product with substantially no stringers in the internal structure. Such an area, if viewed with special instruments, e.g., X-ray diffraction, would depict metallographically a highly uniform structure. Such uniformity results from the use of a wrought metal compsite particle having a high uniform internal structure.

Dispersion strengthened iproducts provided in accordance with the invention are regarded as substantially free from stringers or segregation if they contain less than 10 volume percent of stringers or of regions exceeding 3 microns in minimum dimension in which there is a significant composition fluctuation from the mean, that is to say, a deviation in composition exceeding 10% of the mean content of the segregated alloying element. The boundaries of a segregated region are taken to lie Where the composition deviation from the means is one-half of the maximum deviation in 'that region. Preferably, the minimum dimension of the region of compositional fluctuation does not exceed 1 micron or even 0.5 micron. Preferably also, the proportion of segregated regions is less than 5 volume percent. Compositional variations on the scale discussed 1 above may, for example, be detected and measured by electron microprobe examination.

The wrought composite metal particles which are employed in the starting material are defined in copending application Ser. No. 709,700 as being made by integrating together into dense particles a plurality of constituents in the form of powders, at least one of which is a compressively deformable metal. In one method, they are intimately united together to form a mechanical alloy within individual particles without melting any one or more of the constituents. By the term mechanical alloy is meant that state which prevails in a composite metal particle wherein a plurality of constituents in the form of powders, at least one of which is a compressively deformable metal, are caused to be bonded or united together according to one method by the application of mechanical energy in the form of a plurality of repeatedly applied compressive forces sufficient to vigorously work and deform at least one deformable metal and cause it to bond or weld to itself and/or to the remaining constituents, be they metals and/or non-metals, whereby the constituents are intimately united together. By repeated fracture and rewelding together of said composite particles a fine codissemination of the fragments of the various constituents throughout the internal structure of each particle is ,griding media, e.g., balls, during processing, it appears that the major site at which welding and structural refinement of the product powder takes place is upon the surfaces of the balls.

The process employed for producing mechanically alloyed particles comprises providing a mixture of a plurality of powdered constituents, at least one of which is a compressively deformable metal, and at least one other constituent is selected from the group consisting of a non-metal and another chemically distinct metal, and subjecting the mixture to the repeated application of compressive forces, for example, by agitation milling as one method under dry conditions in the presence of attritive elements maintained kinetically in a highly activated state of relative motion, and continuing the dry milling for a time sufiicient to cause the constituents to comminute and bond or weld together and codisseminate throughout the resulting metal matrix of the product powder. The mechanical alloy produced in this manner is characterized metallographically by a cohesive internal structure in which the constituents are intimately united to provide an interdispersion of cornminuted fragments of the starting constituents. Generally, the particles are produced in a heavily cold worked condition and exhibit a microst-ructure characterized by closely spaced striations.

It has been found particularly advantageous in obtaining optimum results to employ agitation milling under high energy conditions in which a substantial portion of the mass of the attritive elements is maintained kinetically in a highly activated state of relative motion. However, the milling is sufliciently energetic to reduce the thickness of the initial metal constituents to less than one-half of the original thickness and, more advantageously, to less than of the average initial particle diameter thereof by impact compression resulting from collisions with the milling medium, e.g., grinding balls.

As will be appreciated, in processing powders in accordance with the invention, countless numbers of indiparticles to be contacted must be available at the collision ing employed in carrying out the invention in that some site between grinding balls or between grinding balls and the wall of the mill or container, the process is statistical and time dependent.

One of the attributes of the type of high energy workmetals normally considered brittle when subjected to conventional working techniques, e.g., hot or cold rolling, forging, and the like, are capable of being deformed when subjected to impact compression by energized attritive elements in an attritor mill. An example is chromium powder which was found to exhibit cold workability and compressive deformability when subjected to milling in accordance with the method of the invention. compressively deformable metals are capable of exhibiting a time compressive strain (e,,) as determined by the relationship where ln=natural logarithm, t =original thickness of the fragment and t=final thickness of the fragment, well in excess of 1.0, e.g., 1.0 to 3.0 or even much more.

By the term agitation milling, or high energy miling is meant that condition which is developed in the mill when suflicient mechanical energy is applied to the total charge such that a substantial portion of the attritive elements, e.g., ball elements, are continuously and kinetically maintained in a state of relative motion. For optimum results, it has been found advantageous to maintain a major portion of the attritive elements out of static contact with each other; that is to say, maintained kinetically activated in random motion so that a substantial number of elements repeatedly collide with one another. It has been found advantageous that at least about 40%, e.g., 50% or 70% or even or more, of the attritive elements should be maintained in a highly activated state. While the foregoing preferred condition usually does not prevail in a conventional ball mill in which a substantial portion of the ball elements is maintained in static bulk contact with each other, it is possible to employ such mills in carrying out the invention provided there is sufficient activation of attritive elements in the cascading zone and also, provided the volume ratio of attritive elements to the charge in large, for example, 10 to 1 and higher, e.g. 18 to 1.

Since generally the composite metal particles produced in accordance with the invention exhibit an increase in hardness with milling time, it has been found that, for purposes of this invention, the requirements of high energy milling are met when a powder system of carbonyl 7 nickel powder mixed with 2.5 volume percent of thoria is milled to provide within 1-00 hours of milling and, more advantageously, within 24 hours, a composite metal powder whose hardness increase with time is at least about 50% of substantially the maximum hardness increase capable of being achieved by the milling. Putting it another way, high energy milling is that condition which will achieve in the foregoing powder system an increase in hardness of at least about one-half of the difference between the ultimate saturated hardness of the composite metal particle and its base hardness, the base hardness being that hardness determined by extrapolating to zero milling time a plot of hardness data obtained as a function of time up to the time necessary to achieve substantially maximum or saturation hardness. The resulting composite metal particles should have an average particle size greater than 3 microns and, more advantageously, greater than microns.

By maintaining the attritive elements in a highly activated state of mutual collision in a substantially dry environment and throughout substantially the whole mass, optimum conditions are provided for comminuting and cold welding the constituents accompanied by particle growth, particularly with reference to the finer particles in the mix, to produce a mechanically alloyed structure of the constituents within substantially each particle. Where at least one of the compressively deformable metallic constituents has an absolute melting point substantially above about 600 K., the resulting composite metal powder will be heavily cold worked due to impact compression of the particles arising from the repeated collision of elements upon the metal particles. For optimum results, an amount of cold work found particularly useful is that beyond which further milling does not further increase the hardness, this hardness level having been referred to hereinbefore as saturation hardness.

As illustrative of one type of attritive condition, reference is made to FIG. 1 which shows a batch of ball elements 10 in a highly activated state of random momentum by virtue of mechanical energy applied multidirectionally as shown by arrows Hand 12, the transitory state of the balls being shown in dotted circles. Such a condition can be simulated in a vibratory mill. Another mill is a high speed shaker mil-l oscillated at rates of up to 1200 cycles or more per minute wherein attritive elements are accelerated to velocities of up to about 300 centimeters per second (cm./sec.).

A mill found particularly advantageous for carrying out the invention. is a stirred ball mill attritor comprising an axially vertical stationary cylinder having a rotatable agitator shaft located coaxial'ly of the mill with spaced agitator arms extending substantially horizontally from the shaft. A mill of this type is described in the Szegvari U.S;. Pat. No. 2,764,359 and in Perrys Chemical Engineers Handbook, Fourth edition, 1963, at pages 8-26. A schematic representation of this mill is illustrated in FIG. 2 of the drawing which shows in partial section an up standing cylinder 13 surrounded by a cooling jacket 14 having inlet and outlet ports. 15 and 16, respectively, for circulating a coolant, such as water. A shaft 17 is coaxially supported within the cylinder 'by means not shown and has horizontal extending arms 18, 19 and 20 integral therewith. The mill is filled with attritive elements, e.g., balls 21, sufficient to bury at least some of the arms so that, when the shaft is rotated, the ball charge, by virtue of the agitating arms passing through it, is maintained in a continual state of unrest or relative motion throughout the bulk thereof.

The dry milling process of the invention is statistical and time dependent as well as energy input dependent, and milling is advantageously conducted for a time sufficient to secure a substantially steady state between the particle growth and particle comminution factors. If the specific energy input rate in the milling device is not sufficient, such as prevails in conventional ball milling practice for periods up to 24 or 36 hours, a compressively deformable powder will generally not change in apparent particle size. It is accordingly to be appreciated that the energy input level should advantageously exceed that required to achieve particle growth, for example, by a factor of 5, 10 or 25, such as described for the attritor mill hereinbefore. In such circumstances, the ratio of the grinding medium diameter to the average particle diameter is large, e.g., 20 to 50 times or more. Thus, using as a reference a mixture of carbonyl nickel powder having a Fisher subsieve size of about 2 to 7 microns mixed with about 2.5% by volume of less than 0.1 micron thoria powder, the energy level in dry milling in the attritor mill, e.g., in air, should be sufficient to provide a maximum particle size in less than 24 hours. A mill of the attritor type with rotating agitator arms and having a capacity of holding one gallon volume of carbonyl nickel balls of plus inch and minus /2 inch diameter with a ball-to-powder volume ratio of about 20 to 1, and with the impeller driven at a speed of about 180 revolutions per minute (r.p.m.) in air, will provide the required energy level.

The milling time t require to produce a satisfactory dispersion; the agitator speed W (in r.p.m.); the radius r, of the cylinder (in cm.) and the volume ratio R of balls to powder are related by the expression:

where K is a constant depending upon the system involved. Thus, once a set of satisfactory conditions has been established in one mill of this type, other sets of satisfactory conditions for this and other similar mills may be predicted by use of the foregoing expression.

When dry milled under these energy conditions without replacement of the air atmosphere, the average particle size of the reference powder mixture will increase to an average particle size of between about 100 to 125 microns in about 24 hours.

Attritor mills, vibratory ball rnills, planetary ball mills, etc., are capable of providing energy input within a time period and at a level required in accordance with the invention. In mills containing grinding media, it is preferred to employ metal or cermet elements or balls, e.g., steel, stainless steel, nickel, tungsten carbide, etc., of relatively small diameter and of essentially the same size. The volume of the powders being milled should be substantially less than the dynamic interstitial volume between the attritive elements, e.g., the balls, when the attritive elements are in an activated state of relative motion. Thus, referring to FIG. 1, the dynamic interstitial volume is defined as the sum of the average volumetric spaces S between the balls while they are in motion, the space between the attritive elements or balls being sufficient to allow the attritive elements to reach sufficient momentum before colliding. In carrying out the invention, the volume ratio of attritive elements to the powder should advantageously be over about 4 to 1 and, more advantageously, at least about 10 to 1, so long asthe volume of powder does not exceed about one-quarter of the dynamic interstitial volume between the attritive elements. It is preferred in practice to employ a volume ratio of about 12 to l to 50 to 1.

By working over the preferred volume ratio of 12 to l to 50 to 1 on a powder system in which at least one constituent is a cold workable metal, a high degree of cold welding is generally obtained where the deformable metal powder has a melting point above 600 K. In addition, wrought products produced from the powders exhibit highly improved properties. Cold welding tends to increase the particle size and, as the particle size increases, the composition of each particle approaches the average composition of the starting mixture. An indication that satisfactory operating conditions have been achieved is the point at which a substantial proportion of the product powders, e.g., 50% or or or more, have substantially the average composition of the starting mixture.

The deformable metals in the mixture are thus sub-- 9 jected to a continual kneading action by virtue of impact compression imparted by the grinding elements, during which individual metal components making up the starting powder mixture become comminuted and fragments thereof are intimately united together and become mutually interdispersed to form composite metal particles having substantially the average composition of the starting mixture. As the particles begin to work harden, they become more susceptible to attrition so that there is a concomitant building up and breaking down of the particles and a consequent improvement of dispersion. The comminuted fragments kneaded into the resulting host metal particle will generally have a dimension substantially less than that of the original metal powders. Refractory hard particles can be easily dispersed in the resulting particle at interparticle spacings of less than one micron, despite the fact that the starting powder might have been large in size, e.g., up to about 150 microns. In this connection, the disadvantages inherent in other powder metallurgy techniques are overcome.

The product powders produced in accordance with the invention have the advantage of being non-pyrophoric, i.e., of not being subject to spontaneous combustion when exposed to air. Indeed, the product powders are sutficiently large to resist substantial surface contamination when exposed to air. Thus, in general, at least about 75% of the product particles will 'be 10 microns or microns or greater in average particle diameter. The particles generally range in shape from substantially equiaxed to thick flaky particles having an irregular outline and an average low surface area per unit weight, i.e., a surface area not greater than about 6000 square centimeters per cubic centimeter of powder. Because the constituents are intimately and densely united together, there is very little, if any, internal porosity within the individual product particles. The product particles may have a size up to about 500 microns with a particle size range of about 20 to about 200 microns being more common when the initial mixture contains a major proportion of an easily deformable metal, such as an iron group metal, copper and similar deformable metals. The relatively large particle size and low surface area which characterize the composite particles is an outstanding advantage in powder metallurgy processes requiring vacuum degassing for removing adsorbed or absorbed gases. The significance of this advantage becomes particularly marked when it is considered that certain fine metal particles absorb as much as 10 times the volume of gas present in the interstitial spaces between the powder particles. Individual'phases present in the product particle as comminuted fragments derived from constituent particles present in the initial powder mixture retain their original chemical identity in the mechanically alloyed product powder. The individual starting constituents can be identified by X-ray diffraction. The integrity of the mechanically alloyed product particles is such that the hardness thereof can usually be determined on the particles through the use of a standard diamond indenter employed in usual microhardness testing techniques. In contrast thereto, powder particles loosely sintered or agglomerated together by conventional techniques will usually collapse or fragment under the influence of a diamond indenter. The composite product powder produced in accordance with the invention, on the other hand, is characterized by a dense, cohesive internal structure in which the starting constituents are intimately united together, but still identifiable. Such composite particles, because of their compositional uniformity, make excellent building blocks for the production of wrought metal products, such as by hot extrusion of a confined batch of particles.

Referring again to the reference mixture of carbonyl nickel powder mixed with about 2.5 volume percent of thoria (less than 0.1 micron) in an attritor mill, tests have shown that substantial interdispersion and particle growth is achieved when a composite metal particle is produced exhibiting an incerase in cold worked hardness of at least about 50% of the difference between the ultimate saturation hardness of the product particle and the base hardness of the composition as determined by extrapolating hardness data to zero grinding time. More preferably, it has been found advantageous that the microhardness of thecomposite metal powder be at least about 75% of the difference between the ultimate saturation hardness and the extrapolated hardness.

Referring to FIG. 3, two curves are shown relating Vickers microhardness to time of milling as determined for-two types of grinding mills. Both hardness curves A and B were obtained by dry milling a charge of carbonyl nickel powder having an average Fisher subsieve size of 3 to 5 microns mixed with thoria having a particle size of less than 0.1 micron, except that curve A was obtained by milling the mixture at an 18 to 1 volume ratio of ballsto-powder charge in a high energy stirred ball mill attritor of the type shown in FIG. 2, while curve B was obtained by milling-the same mixture, with the same ball-to-charge ratio, in a lower energy ball mill. Referring to curve A, it will be noted that a saturation hardness of approximately 650 Vickers is achieved after about 16 hours of dry milling in the high energy stirred ball mill attritor; whereas, with respect to curve B (a lower energy ball mill with an 18 to 1 ball-to-charge ratio), approximately the same saturation harness of about 650' Vickers is achieved after about 190 hours of dry milling. It will be noted that both curves extrapolate to a base hardness at zero time of above about 300 Vickers. It will be further noted from the two curves that about one-half of the hardness increase is achieved in the case of the high energy mill (curve A) in about Shows, and in the case of the lower energy mill (curve B) in about hours, 75% of the hardness increase being achieved in the mills in about 10 hours and hours, respectively. As stated hereinbefore, the requirement of high energy milling as applied to the foregoing nickel-thoria system as a reference, is met when a wrought composite metal power of the system can be produced in about 100 hours and, more advantageously, within about 24 hours, having a hardness increase of at least about 50% of substantially the maximum hardness increase capable of being achieved by dry milling for that reference system. In the ball mill run with a ball-topowder ratio of 18 to 1, the data are given on the basis of loose powder found in-the mill at the end of the run.

While the saturation hardness in curve A remains substantially constant from about 16 to over hours, further changes may take place in the composite metal powder druring milling beyond 24 hours. For example,

after saturation or maximum hardness is reached and,

likewise, the maximum particle size, further grinding of the composite metal particles improves the homogeneity in that the intimately united constituents tend to be finer and more closely spaced.

When the initial metal particles have melting points of at least 600 K. and, more preferably, at least 1000" K., substantial cold working of the resulting composite or cold welded particles is found to result from the reduction in thickness. This cold working effect promotes fracture and/ or comminution of the cold welded particles by action of the milling media. Thus, particles of larger size in the initial mixturesare comminuted or reduced in size. Cold welding of particles, both of original particles and cold welded particles occurs with accumulation of material on the particles being milled and on the grinding balls. This latter factor contributes to desired particle growth and theoverall comminution and/or fracture of cold welded particles contributes to size reduction of the particles. As the dry milling proceeds, the average particle size of the milled particles tends to become substantially stabilized with a decrease in both the amount of subsize particles and the amount of oversize particles and with continued refinement of the internal structure of individual milled particles. Individual components of the powder mixture being milled become comminuted and 1 1 fragments thereof become intimately united together and dispersed through the matrix of the product powder. The net result of the complex milling process is a destruction of the original identity of the metal powders being milled and the creation of new composite product powders; however, the original constituents are still identifiable. The product powder particles comprise comminuted fragments of the initial metal powders welded or metallurgically bonded together, with the dimension across the comminuted fragments being usually less than one-fifth or preferably less than one-tenth the average diameter of the initial metal powder from which the fragment was derived, e.g., less than 10 microns or less than 5 microns or even less than 1 micron, e.g., 0.1 or 0.2 or 0.5 to 1 micron. Refractory particles included in the initial powder mixture become mechanically entrapped in and distributed throughout the individual product powder particles in a fine state of dispersion approximately equal to the minimum dimension of the aforementioned fragments. Thus, the refractory particle inte-rparticle distance is much less than the particle diameter of the initial metal powder and can be less than 1 micron, or 0.5 or 0.2 or 0.1 micron in which case there are essentially no dispersoid-free islands or areas.

Again, with reference to powder mixtures having metallic components melting over 600' K. or higher, and preferably at 1000 K. or higher, it is found that the cold working during milling greatly exceeds that obtained in the same metal cold Worked by other means, e.g., by rolling, forging, and the like, to reduce the thickness 90% without intermediate annealing. Thus, a pellet of carbonyl nickel cold reduced 90% by compression with grinding of cracked edges between reduction stages was found to have a hardness of about 250 Vickers whereas carbonyl nickel powder at about 5 micron size dry milled in an attritor mill, i.e., a stirred ball mill of a one gallon size with a charge of carbonyl nickel pellets about A inch in diameter andan impeller speed of 176 rpm. and a ball-to-powder ratio of 18 to 1 exhibited a saturation hardness of 475 Vickers as determined after 24 hours milling time. The Vickers hardness readings obtained on powders as described herein represent the. average of reliable readings obtained on dense particles mounted in 3 standard microspecimen mounting plastic and polished It is to be appreciated that the saturation hardness for each system dry milled in accordance with the invention will be characteristic thereof and is dependent upon composition. Systems containing refractory compound particles have substantially higher saturation hardnesses than the same system devoid of, or substantially devoid of, such particles. For example, the saturation hardness of a nickel-2,5 volume percent thoria system was determined to be about 640 to 650 Vickers hardness as against a Vickers hardnessof about 475 for the same nickel without thoria. I

As applied to a dispersion-hardened system comprising carbonyl nickel powder mixed with 2.5 volume percent of fine thoria, dry milled in the attritor mill using an 18 to 1 ball-to-powder ratio, the cold working effect has been traced by means of X-ray line broadening (CuK f radiation) in which the width of the 111 peak for nickel was measured at one-half the height. Saturation was observed after 20 hours milling time at a 5 value 20) of about 0.5 determined from the formula /8= /B 2 wherein B is the peak width at half height for the nickel-thoria system and b is the peak width at half height for the same nickel powder unprocessed and without thoria. The foregoing technique may be advantageously employed in tracing the cold working effect of dry milling on the refractory metals, low alloy steels, gold-base metals, and the like.

It is important that the milling process be conducted in the dry state and that liquids be excluded from the milling environment since they tend to prevent cold welding and particle growth of metal powder and preventinclusion of fine refractory dispersoids in the product powder. The presence of liquid ingredients in the powder mixture being milled, e.g., water or organic liquids such as methyl alcohol, liquid hydrocarbons, or other liquids, with or without surface active agents such as stearic acid, palmitic acid, oleic acid, aluminum nitrate, etc., effectively inhibits welding and particle growth, promotes comminution of the metal constituents of the mix, inhibits production of composite particles and prevents entrapment of fine dispersoid particles in the product powder. Moreover, wet grinding tends to promote the formation of flakes which should be avoided. The fine comminuted metal ingredients also tend to react with the liquid, e.g., alcohol, and the greatly increased surface area resulting inhibits extraction of absorbed gas under vacuum. Generally, very fineparticles tend to be produced which are susceptible to contamination on standing in air or may even be pyrophoric. A virtue of dry milling is that in many cases, air is a suitable gas medium. Alternatively, nitrogen, hydrogen, carbon dioxide, argon and helium and mixtures of these gases can also be employed. When the inert gases argon and helium are employed, care should be taken to eliminate those gases from the product powder mixture prior to final consolidation thereof by powder metallurgy methods. Inert gas media tend to enhance product particle growth and may be of assistance when powder mixtures containing active metals such as aluminum, titanium, etc are being milled. Preferably, the milling temperature does not exceed about 350 F. Generally, the temperature is controlled by providing the mill with a water-cooled jacket such as shown in FIG. 2.

The invention is applicable to the treatment of a wide variety of metal systems having starting particle sizes ranging'from about 2 microns to about microns and higher. The particles should not be so fine as to be pyrophorically active. Coarse particles will tend to break down to smaller sizes during the initial stages of dry milling while particle growth occurs during formation of the composite metal particle.

As stated hereinbefore, the powder mixture may comprise a plurality of constituents so long as at least one of which is a metal which is compressively deformable. In order to achieve the results of the invention, the ductile metal should comprise at least about 15%, or 25%, or 50% or more by volume of the total powder composition. Where two or more compressively deformable metals are present, it is to be understood that these metals together should comprise at least about 15% by volume of the total powder composition.

In producing wrought or heavily cold worked composite metal particles from the broad range of materials mentioned hereinbefore, the starting particle size of the starting metals may range from about over 1 micron up to as high as 1000 microns. It is advantageous not to use too fine a particle, particularly where reactive metals are in volved. Therefore, it is preferred that the starting particle of the metals range from about 3 microns up to about 200 microns.

The stable refractory compound particles may, on the other hand, be maintained as fine as possible, for example, below 2 microns and, more advantageously, below 1 micron or even below 0.1 micron. A particle size range recognized as being particularly useful in the production of dispersion strengthened systems is 10 angstroms to 1000 angstroms (0.001 to 0.1 micron).

In working with metals which melt above 1000" K., the heavy cold work imparted to the composite metal particle is particularly advantageous in the production of alloys and dispersion strengthened metals and alloys. Observations have indicated that the heavy cold work increases effective diffusion coeificients in the product powder. This factor, along with the intimate mixture in the product powder of metal fragments from the initial components to provide small interdiffusion distances, promotes rapid homogenization and alloying of the product powder upon heating to homogenizing temperature. The foregoing factors are of particular value in the production of powder metallurgy articles having rather complex alloy matrices.

One of the advantages of formulating compositions in accordance with the invention is that very little or no oxidation occurs during high energy milling. Generally, the extraneous oxides which appear in the final consolidated products are principally those present in the starting material. However, unlike the kind of oxidation which occurs in conventional melting techniques, these extraneous oxides appear as fine dispersoids and can be useful as dispersion strengtheners, provided they are chemically stable and temperature resistant.

In the production of alloyed powders for usual powder metallurgy purposes, amounts of oxygen up to about 1% by weight, preferably not exceeding about 0.5% oxygen may be tolerated. In the present invention, such oxygen may be availed of to produce dispersion hardening by internal oxidation of metals having a free energy of formation of the oxide exceeding 90 kg. cal. per gram atom of oxygen milled into the mechanically alloyed powder upon heating of the powder due to the short diffusion distances involved.

It is to be appreciated that the dispersoid employed in producing dispersion-strengthened products according to the invention can be selected from a wide variety of refractory compounds having regard for the stability of the material in the metal matrix involved as those skilled in the art will understand. Thus, the refractory compound may be an oxide, carbide, nitride or boride of a refractory metal such as thorium, zirconium, hafnium, or titanium, an oxide of a metal such as silicon, aluminum, yttrium, cerium, lanthanum, a rare earth mixture known as didymium, magnesium, calcium, beryllium and the like. Certain beryllides and aluminides stable in the metal matrix em: ployed may also be used. In metals such as nickel and copper, yttria, lanthana, ceria, thoria or didymia may be used. In zinc-base materials, alumina, zirconia'and titania may be used. In steels, alumina, zirconia, yttria, lanthana, didymia and titania are satisfactory. In columbium, tantalum, tungsten and molybdenum and their alloys, thoria, yttira, lanthana and didymia may be employed. These latter oxides may also be used in platinum and gold and their alloys when higher operating temperatures are contemplated although alumina, zirconia and titania may be satisfactory if lower temperatures are contemplated.

Dispersion strengthened nickel In order to give those skilled in the art a better understanding of the invention with respect to the dispersion hardening of nickel, the following illustrative example is given:

EXAMPLE -I A charge consisting of 1,173 grams of carbonyl nickel powder having an average Fisher subsieve size of 3 to 5 microns with 27 grams of thoria having a particle size of about 50 angstroms was preblended in a high speed food blender (Waring Blender) and was then dry milled in air for 24 hours in a stirred vertically upstanding, water-jacketed attritor mill of the type illustrated schematically in FIG. 2. The mill contained a one-gallon volume of carbonyl nickel shot or balls of average diameter of about one-quarter inch and operated at an impeller speed of about 176 r.p.m. The volume ratio of the ball to the powder charge was 18 to 1. Upon completion of the milling, the powder Was separated from the attritive elements and occasional coarse particles removed from the powder. The powder which had a saturation hardness of about 640 to 650 Vickers was placed in a mild steel extrusion can and evacuated to a pressure of less than 0.1 micron of mercury at 400 C. The can was then sealed, heated to 1800 F. and extruded at an extrusion ratio of ,16' to 1. The extruded product contained a fine, uniform, stable dispersion of thoria. Light 14 microscopy and electron microscopic examination of surface replicas revealed that the metal grains in the metal were less than '5 microns in size with the microscopic examination of surface replicas revealed that the metal grains in the metal were less than 10 microns in size and that the thoria particle was less than 0.2 micron. The structure of a particle of the thoriated nickel powder in the as-milled condition is depicted in FIG. 4

Hot ultimate tensile strength (p.s.i.) as extruded and after cold swaging Test (percent RA.)

temperature,

F. As-extruded ,40% 65% Norn.-P.s.i.=Pounds per square inch; R.A.=

Reduction in area.

In contradistinction to the foregoing, a charge consisting of 977.5 grams carbonyl nickel powder having an average Fisher subsieve size of 3 to 5 microns with 22.5 grams of thoria having a particle size of about 50 angstroms was dry milled in air for six hours in the same mill as in Example I using a running speed of 146 r.p.m. Four batches were prepared in the same manner at a ball-to-powder weight ratio of 22 to 1 and 2,000 grams of the product accumulated from which occasional coarse particles were removed. The powder was similarly hot extruded under the same conditions as in Example I. The extruded product contained a fine, uniform, stable dispersion of thoria. Light microscopy and electron microscopic examination of surface replicas revealed that the metal grains in the metal were less than 10 microns in size and that the thoria particle was less than 0.2 micron. The structure of a particle of the thoriated nickel powder in the as-milled condition is depicted in FIG. 4 of the drawing. The average particle size of the milled powder was less than 74 microns. A portion of the powder depicted in FIG. 4 was annealed in argon for 16 hours and the structure of an annealed particle is depicted in FIG. 5, wherein the fine grained areas are those in which a good thoria distribution was achieved in the six hour milling time. In these areas the grain sizes are less than 1 micron. The areas wherein poor distribution of thoria was obtained due to the short miling time and low milling speed, the grain size is on the order of 10 microns. The dark spots and lines are holes associated with carbon and oxygen impurities in the starting material. A portion of the as-extruded material was cold worked 22% by bar rolling and the properties of the as-extruded and cold worked material were determined by the short-time tensile test at various temperatures with the following results:

TABLE II Yield Test strength Tensile temp., 0.2% ofiset, strength, EL, R.A:, F. Condition k.s.i. k.s.i. percent percent Room. As extruded... 92. 9 110. 5 19.0 60. 5 1,400 o 14.1 14.5 14.0 38.5 1,400 Cold worked.- 16.7 17.6 15.5 29.0 1,800 As extruded... 5.4 8.7 10.9 11.6 1,800 Cold workecL- 7.3- 13.6 7.6 11.6 2,000 As extruded..- 4.8 6.9 9.0 16.0

NOTE.-K.S.1. =Thousands of pounds per square inch; El. =Elongation As 'a'further comparison, the procedure of Example I was repeated using a 2,200 gram batch of powder with a milling time of' 24 hours in air in the same attritor mill running at a speed of 184 r.p.m. The ball-to-powder volume ratio in this. instance was about 10 to 1. Material from this batch was hot extruded and the structure of the as-extruded material is depicted in FIG. 7 taken at 750 diameters. The internal structure of the composite metal particle wassimilar to that obtained in Example I in that the. thoria-free areas which were depicted in FIG. 6 were eliminated. The as-extruded tensile strength of this material at 2000 F. was 7,400 pounds per square inch (p.s.i.). A portion of the as-extruded material was then cold swaged to reduce the cross sectional area thereof 75%. After the cold swagging treatment, the tensile strength of the material at 2000" F. was raised to 15,000 p.s.i. The structure. of ,the as-extruded cold swaged material at 750 diameters is depicted in FIG. 8 of the drawing.

The Cb-base, Ta-base, Mo-base and W-base refractory metals The refractory metals are of particular interest as structural elements for airframes and rocket motors. Sheet products have been proposed for re-entry glide vehicles and ramjet and turbojet engines. Machined forgings have been used for rocket nozzles. As applied to the leading edge sections, such as those on a re-entry glide vehicle, the refractory metal may serve as a hot structure, capable of supporting aerodynamic and thermal stresses. In this connection, dispersion strengthening would improve the high temperature and high strength utility of such metals. An example of one alloy composition that may be dispersion strengthened is 10% molybdenum, 10% titanium and the balance essentially columbium. Another refractory-base material is one containing about 90% tantalum and about 10% tungsten. By adding 2% volume percent ThO to such alloys, the'strength properties can be improved. With respect to high strength tungsten and molybdenum containing 2 volume percent ThO fabricability and low-temperature ductility are not diminished, while strength at elevated temperatures, e.g., particularly creep resistance, is greatly enhanced. The term refractory-base metal as applied to columbium, tantalum, molybdenum and tungsten is meant to cover these metals per se (substantially 100%) as well as alloys based on these metals, the refractory metal constituting at least about 45% f the alloy exclusive of the dispersoid.

In order to give those skilled in the art a better understanding of the invention, the following illustrative examples are given:

EXAMPLE II A charge consisting of about 2,160 grams of tantalum and 240 grams of tungsten to produce a total charge of 2400 grs.. (90%Ta-10% W) is provided to which is addedabout 28 grams of thoria of about 0.02 micron in average size to provide a volume loading of approximately 2%. The particle size of the metals range from about 3 to 40 microns. The charge is pr'eblended in a high speed food blender (Waring Blendor) and is then dry milled for 40 to 50 hours in the stirred vertically have reached saturation hardness.

The composite powder, which has an average composition of about 90% tantalum and tungsten with a 1 6 volume loading of approximately 2%- ofthoria is prepared for hot Working by packing in a molybdenum-can 3 inches in diameter. The can is evacuated to-less than 10 mm. Hg at 950 F., sealed by fusion welding and consolidated by hot extrusion to inchrod at 2400 F. The wrought tantalum-tungsten alloy produced by the foregoing method Will exhibit a highly uniform dispersion of thoria in both the longindinal and transverse cross section.

EXAMPLE III In producing dispersion strengthened 'columbium, about 1100 grs. of-columbium powder of about 10 to 50 microns in average size is preblended with about 26 g. of thoria of about 0.04 micron in size. The blended mix is placed in the mill described in Example II containing about one-gallon volume of one-quarter inch tool steel balls and the mill similarly operated at an impeller speed of about 176' r.p.m., the volume ratio of balls to charge being about 18 to 1. Upon completion of milling after 48 hours, the powder is removed sieved and the sieved powder then subjected to hot working by packing in a 3 /2 inch diameter molybdenum. can, evacuating the can to 10 mm. Hg at 950 F., and sealed by'fusion welding. The can is then heated to 2700 F. in hydrogen and quickly transferred to the extrusion press where it is extruded to 1 inch bar. The resulting consolidated bar displays a uniform distribution of fine thoria in a dense matrix.

EXAMPLE IV Dispersion hardened tungsten is produced by'preblending 2500 grams of tungsten powder (10 microns average particle size) with 27 grams of thoria of about 0.02 micron average size to provide a volume loading of about 2 volume percent of thoria. The mixture is milledunder the same conditions as that set forth in Example III for about 40 to 50 hours following which the composite particles are removed and sieved and the sieved powder prepared for hot working by packing in a 3% inch diameter molybdenum can, evacuating the can to 10- mm. Hg at 950 F. and sealing the can by fusion welding. The can is then heated to 3500" F. in hydrogen and quickly transferred to the extrusion press where it is extruded to 1 inch diameter bar. Again, a uniform distribution of ifine thoria is obtained in a consolidated tungsten matrix.

In a parallel procedure using molybdenum powder of about 20 microns average particle size, thoriated molybdenum is also produced using an extrusion temperature of 3500 F.

Dispersion strengthened low alloy steels Low alloy steels which may be produced in accordance with the invention include those steels having a composition encompassed by the range by weight of up to about 1. 1% carbon, for example 0.25 to 1.1% carbon, e.g., 0.09 to 0.8% carbon, at least 0.15% of an alloying metal from the group consisting of up to about 5% chromium and up to about 5% molybdenum, and the balance essentially iron. Other ingredients which may be present include up to about 2% vanadium, up to 2% tungsten, up to 5% nickel, up to about 2% silicon, and up to about 2% manganese, among others. Examples of well known low carbon steel compositions are given as follows:

TABLE III Nominal composition by weight, percent Alloy steel No. C Cr Mo Fe Others 0.08 5 0.5 balauce. 0.5 Ti 0,12 5 0. 5 balance. 1.2 Si 0.15 0.5 balance..- 0.17 0.5 0.5 balance 0.12 1 0.5 balance"- 0.13 0.6 0.01 balance-.- 0.65 Mn: 0.018 P 0.08 1.25 0.5 balance-.- 0.06 Zr 0.18 2 1.0 balauce 0.12 2.25. 0.5 balance-" 0.4 2 0.35 ba1ance' 0.4 1 balance..- 025V At temperatures up to about 600 or" 700 F, the strength of the foregoing type steels is generally related to the room temperature tensile properties. The eifects of alloying elements become important above the aforementioned temperature range. The addition of molybdenum contributed to creep strength. Thus, adding dispersoids to the foregoing enables the production of low alloy steels having high temperature strength, including high creep strength.

EXAMPLE v In producing a dispersion strengthened low alloy steel containing 2% chromium, 1% molybdenum, 0.4% carbon and the balance essentially iron, except for the dispersoid, a master alloy is first produced containing about 30% chromium, about 15% molybdenum, about carbon and the balance iron. The master alloy which is quite brittle is ground to pass 200 mesh and 80 grams are preblended with 1120 grams of sponge iron of minus 100 mesh (approximately 65 microns) to give a total of 1200 grams corresponding in composition to a low alloy steel containing about 2% chromium, about 1% molybdenum, about 0.35% carbon, and the balance essentially iron. To the powder mixture is added about 30 grams of a rare earth oxide mixture of average particle size of about 0.02 micron and the whole placed in the attritor mill described in Example III and milled under the same conditions for 40 to 50 hours using a one-gallon volume of nickel balls of one-quarter diameter at an impeller speed of about 230 r.p.m. Following completion of milling, the powder is separated from the balls by sieving and a batch of the sieved powder then packed in a 3 /2 inch diameter mild steel can which is then evacuated to mm. Hg at 750F. and sealed by fusion welding. Consoldation was accomplished by heating the sealed can to 1800 F., and extruding to inch diameter bar. A dispersion strengthened steel results which contains a welldistributed dispersion of finely-divided rare earth oxide (principally lanthana) having a particle size of about 0.02 micron and an interparticle spacing of less than 0.5 micron.

EXAMPLE VI Recently a new type of ultra high strength nickel alloy steel has been developed which is referred to as a maraging steel. This steel is unique in that in the solution annealed state, the microstructure is soft martensite, the steel being hardenable merely by heating it to a relatively low temperature of about 900 F. It would be desirable to supplement the age hardening effect with dispersion strengthening. Broadly speaking, the nickel alloy steel may have a composition falling in the following range by weight: about 10 to 30% nickel, about 0.2 to '9%- titanium, up to about 5% aluminum, the sum of the titanium and aluminum not exceeding about 9%, up to about 25% cobalt, up to about 10% manganese, up to about 10% molybenum, substantially the balance of the matrix being at least about 50% iron. An example of a specific composition is a steel containing about 18.5% nickel, 7.5% cobalt, 4.8% molybdenum, about 0.1% aluminum, about 0.40% titanium, and carbon not exceeding about 0.03% maximum. It would be desirable-to produce dispersion strengthened maraging steels by poW- der metallurgy. A difficulty is the rather sluggish diffusivity characteristic of molybdenum when a simple powder mixture of the foregoing ingredients is consolidated and hot extruded. This difiiculty and other difiiculties associated with segregation are obviated by using the method of the invention. A preblended mixture consisting of 480 grams of molybdenum passing 325 mesh, 750 grams of cobalt powder of 4 to 7 microna average size, 1503 grams of nickel powder of 4 to 7 microns average size, 450 grams of a nickel-14% aluminum-8.9% titanium master alloy powder crushed to pass 200 mesh and 6817 grams of iron powder of about 65 microns average size is 18 placed in a high energy attritor similar to that-of Ex' ample I, but containing essentially 10 gallons of hardened steel balls of Mt inch diameter. The mixture is dry milled for 30 hours at an impeller speed of 180 r.p.m. inan atmosphere of nitrogen. The resulting composite powder is substantially homogeneous in microstructure containing all the aforementioned alloying ingredients and occluded oxygen in a fine state of admixturefwhich oxygen will combine with sutficient of the copresent aluminum to form about 2 volume percent of-submicron alumina on heating for extrusion. The product powder will generally have a particle'size such that at least about 75 of the particles exceeds 10 microns in size and are non-pyrof phoric. The product is vacuum packed in a mild steel can, heated to about 1800 F., and hot'extruded at an extrusion ratio of about 12 to 1. By employing a dispersoid in the composition, a hot'extruded' dispersion strengthened product is provided having improved strength properties at a temperature range of about 900 1 .,1to 1200 F. i

Dispersion strengthened zinc-base metals Wrought zinc and zinc alloysare employed in the form of strip, sheet extruded rod, wire, etc. These metals provide resistanceto corrosion for many .types of service. Like other metals and alloys wrought zinc has the char-' acteristic'of flowing under constant loads below the ultimate strength. Dispersion strengthening will be desirable where resistance to creep is a requirement.

Examples of zinc alloys to which the invention is applicable are the following: 0.15 to 0.35% lead, 0.017% iron max., 0.15 to 30% cadmium, 0.005% copper max. and the balance essentially zinc; 0.005 to 0.1% lead, 0.012% iron max., 0.05% max. cadmium, 0.5 to 1.5% copper, 0.12 to 1.5% titanium, and the balance essentially zinc; up to 0.025% magnesium, 0.25 to 0.6% aluminum, and the balance essentially zinc; upto 35% copper, 0.02 to 0.1% magnesium, 3.5 to 4.5% aluminum; and the balance essentially zinc. The latter alloy is particularly noted for its high strength and hardness. Zinc or zinc alloys containing at least about 50% zinc, with the remainder one or more metals from the group lead, tin and cadmium can be dispersion strengthened in accordance with the invention. 1

EXAMPLE VII In producing dispersion hardened zinc using 0.02 micron gamma alumina as the dispersoid, 1500 grams of minus mesh zinc powder is preblended with 25 grams of gamma alumina. The preblended powder is milled in the attritor mill of FIG. 2 as described in Example *I, onequarter inch nickel balls being used sufficient to provide a volume ratio of balls to charge of about 20 to 1. The charge is milled for about 60 hours at an impeller speed of about 180 r.p.m., following which the milled charge is removed and separated by sieving. The sieved powder is then cold hydrostatically pressed and sintered at 600 F. in dry hydrogen. The 2 /2 inch diameter billet'resulting is machined smooth and consolidated by extrusion to inch diameter rod at 350 F. The consolidated, extruded .rod will exhibit a highly uniform dispersion of Dispersion strengthened platinum-base metals Platinum and its alloys have a wide diversity of uses. Platinum .is quite soft and does not tend to have good resistance to creep. Alloys of platinum and rhodium are particularly useful at high temperatures under oxidizing conditions and as electrical contacts. Dispersion strengthening of platinum-base metals is particularly desirable. Besides pure platinum, examples of other platinum-base metals are the following: 'up to about 50% palladiumfi criteria set forth here- 19 and the balance essentially platinum: about 3.5 to 40% rhodium, and the balance essentially platinum; up to 35% iridium, and the balance essentially platinum; up to 8% tungsten, and the balance essentially platinum. Examples of dispersion strengthened platinum-base metals which can be produced as wrought shapes in accordance with the invention are as follows: a wrought platinum containing 2 volume percent of 0.02 micron thoria; a wrought alloy of 75% platinum-25% rhodium containing 3 volume percent of 0.04 micron yttria; a wrought alloy of 92% platinum-8% tungsten containing volume percent of 1 micron titanium carbide; and a wrought alloy of 90% platinum and 10% palladium containing 2 volume percent of 0.1 micron zirconia.

Dispersion strengthened gold-base metals Gold is quite soft and has low resistance to creep. Gold can be hardened by the addition of alloying elements. However, gold-base metals are particularly suited to dispersion strengthening in accordance with the invention by using the same methods set forth in Examples I to VII. Besides gold, the gold-base metals may include 54 to 60% gold, 14 to 18% platinum, 1 to 8% palladium, 7 to 11% silver, 11 to 14% copper, 1% max. nickel and 1% max. zinc; 62 to 64% gold, 7 to 13% platinum, 6% max. palladium, 9 to 16% silver, 7 to 14% copper and 2% max. zinc; 70% gold and 30% platinum; among others. Volume loadings of up to 10% of dispersoid maybe produced in wrought products of gold-base metal in accordance with the invention, for example, volume loadings of 1%, 2%, 4%, 8% and 10% of thoria, yttria, alumina, refractory carbides, etc.

Dispersion strengthened copper Copper can be dispersion strengthened to greatly improve the physical properties, such as resistance to creep at elevated temperatures, while still maintaining relatively high electrical and thermal conductivities. In this respect, the following example is given:

EXAMPLE VIII A charge consisting of 1,173 grams of copper powder having an average Fisher subsieve size of about 7 to 10 microns with 27 grams of alumina having a particle size of about 0.03 micron and is then dry milled in air for 30 hours in a stirred vertically upstanding water-jacketed attritor mill of the type illustrated schematically in FIG. 2 containing one-gallon volume of hardened steel balls of average diameter of about one-quarter inch and operating at an impeller speed of about 176 rpm. The volume ratio of the ball to the powder charge is 18 to 1. Upon completion of the milling, the powder is separated from the attritive elements. The powder is hydrostatically compressed into the shape of a billet and the billet is then sintered in hydrogen at 850 C. for one hour. The sintered billet is vacuum welded within a copper can and the can is hot extruded at an extrusion ratio of 1 8 to l at a temperature of about 800 C. to produce a wrought copper product substantially free from stringers, characterized by high electrical and thermal conductivities, and having strength at ambient and elevated temperatures substantially above that of pure copper.

In like manner, dispersion-strengthened copper-base alloys such as brasses, bronzes, nickel silvers, beryllium copper, cupronickels, etc., can be produced which have substantially improved strength at ambient and elevated temperatures as compared to the non-dispersion-strengthened base alloy. Such copper-base alloys may contain up to 35% zinc, up to 50% nickel, up to 10% tin, up to 10% aluminum, up to 3.5% silicon, up to 2.4% beryllium, up to 3.75% lead with minor additions such as up to about 1.35% manganese, up to about 0.4% phosphorus, up to about 0.05% arsenic and up to about 0.45% tellurium. Dispersoids such as alumina, zirconia, yttria, lanthana,

silica, thoria, etc., may be employed in these alloys. The 1 20 alloys may 'beprepared .by dry milling as described in the foregoing example, with consolidation of the powders being performed, for example, by extrusion at temperatures of 500 C. to 1000 C., e.g., 650 C.

It is to be understood that solid solution alloys such as alloys in the nickel-copper series, e.g., 70% nickel-30% copper, may readily be prepared in accordance with the invention. 1

'It will be apparent from the foregoing that numerous advantages are provided by the invention. For example, a difficulty frequently met with in powder metallurgy in producing wrought metal products is the thorough mixing of small amounts of one constituent, such as a dispersoid, with large amounts of another, such as a matrix metal. The present invention enables this to be done easily and without segregation occurring subsequent to the mixing and .during hot working.

Because the mixing occurs in effect by mechanically alloying the constituents together within separate particles, interparticle spacing between constituents is fixed and predetermined leading to vastly improved and rapid homogenization by means of short-time diffusion annealing treatments. In addition, reactive components, e.g., chromium, aluminum, titanium, and the like, are in effect neutralized by the milling technique by being incorporated into and being protected by the matrix of the host metal, e.g., iron, making up the major constituent of the composite metal particle.

No matter how coarse the product powder produced in accordance with the invention, the dissemination of the constituents in the particle is extremely intimate and fine. The advantage of providing a coarse composite metal powder is that it can be stored with minimum contamination, is capable of being easily outgassed for canned-extrusion, is nonpyrophoric, has good flow characteristics and exhibits relatively high apparent or tap density.

The Wide and different ranges of compositions capable of being produced in accordance with the method of the invention are almost boundless. The extent of limited liquid or solid solubility of the constituents does not present the problems which are normally confronted with conventional alloying techniques. Thus, alloys of a wide range of complexity are possible by means of the invention. The recovery of chemical constituents making up the composition is more predictable than by conventional melting techniques and the constituents are not subject to separation during mechanical handling.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

-I claim:

1. As a powder metallurgy article of manufacture, a wrought, dispersion strengthened, consolidated metal product produced from mechanically alloyed metal powder selected from'the group consisting of nickel; copper; copper alloys; low alloy steels; maraging steels; zinc-base metals; the columbium-base, tantalum-base, molybdenumbase and tungsten-base refractory metals; platinum-base metals, and gold-base metals, characterized by a'uniform dispersion of about 0.05 to 25 volume per cent of a refractory compound dispersoid in both the longitudinal and transverse direction such that said consolidated metal product contains less than 10 volume percent of segregated regions exceeding 3 microns in minimum dimension.

2. The powder metallurgy article of maufacture of claim 1, wherein the dispersoid is stable in the metal matrix and is selected from the group consisting of yttria, lanthana, alumina, titania, zirconia, didymia and thoria ranging from about 0.05 to 10 volume percent at an average particle size of about 10 to 1000 angstroms and at an average interparticle spacing of less than one micron, and wherein the metallographic structure thereof is substantially free from dispersoid stringers over any selected area taken in longitudinal or transverse section of up to about 500 microns in average diameter such that said consolidated metal product contains less than volume percent of segregated regions exceeding 1 micron in minimum dimension.

3. The powder metallurgy article of manufacture of claim 2, wherein the formulated composition of the wrought metal product is nickel containing about 0.05 to volume percent of the dispersoid.

4. The powder metallurgy article of manufacture of claim 2, wherein the wrought metal product is copper characterized by high electrical and thermal conductivities. 1

5. The powder metallurgy article of manufacture of claim 2, wherein the formulated composition of the wrought metal product is nickel containing about 0.05 about 0.05 to 10 volume percent of the dispersoid, and the dispersoid is selected from the group consisting of alumina, zirconia, titania, yttria, lanthana and didymia.

6. The powder metallurgy article of manufacture of claim 2, wherein the formulated composition of the wrought metal product is a maraging steel containing about 0.05 to 10 volume percent of the dispersoid, and the dispersoid is selected from the group consisting of alumina, zirconia, titania, yttria, lanthana and didymia.

7. The powder metallurgy article of manufacture of claim 2, wherein the formulated composition of the wrought metal product is a zinc-base metal containing about 0.05 to 10 volume percent of the dispersoid, and

the formulated composition of the wrought metal product is a platinum-base metal containing about 0.05 to 10 volume percent of the dispersoid.

10. The powder metallurgy article of claim 3, wherein the formulated composition of the wrought metal product is a gold-base metal containing about 0.05 to 10 volume percent of the dispersoid.

References Cited UNITED STATES PATENTS 3,159,908 12/1964 Anders 206 X 3,143,789 8/1964 Iler et a1. 75206 X 3,179,515 4/1965 Grant et a1. 75206 3,176,386 4/1965 Grant et a1 75206 X 3,591,349 7/1971 Benjamin 75203 X 3,623,849 11/1971 Benjamin 75-182.8

BENJAMIN R. PADGE'IT, Primary Examiner R. E. SCHAFER, Assistant Examiner U.S. C1. X.R. 75206 mg? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 8' Dated June 1973 Inventorls) JOHN STANWO-OD BENJAMIN It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 4, line 49, for "pisted" read -picted-.

Line 73, delete "a".

Col. 6, line 15, after "milling" insert --need. not be limited to such conditions so long as the milling. Line 40, for "time" read true. Line 69, for "in" read --is-.

Col. 7, line 55,- for "pages" read --page I Col. 8, line 22, for "require" read --reguired--.

Col. 9:, line 75, for "incerase" read -incr'ease-.

Col. 10, line 49, for "druring" read -during.

Col. 11, line 53, for "2,5" read -2.5--.

Line 65, delete the formula and insert the following corrected formula: 2 2

Col. 12, line 24, for "those" read --these.

Col. 14, line 3, after "the" delete the remainder of the. paragraph and insert the following: -bulk of the grains being less than 1 micron and that .the thoria particle size was less than 0.2 micron with most of the thoria being about 200 Angstroms. The structure of a particle of the thoriated nickel powder in the as-milled condition was similar to that! depicted in Fig. 4 of the drawing.

@7 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3'738'8l7 Dated June 1973 inventor) JOHN STANWOOD BENJAMIN It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Page 2 Col. 14, Table I, for column heading "65%" read --6l%-.

, Line 48, for "The" read -In-. Line 49, for "miling" read milling-.

Table II, under column heading "Tensile strength, ksi", for "14.5" read -l4.l-. Under column. heading "El. percent" for 7.6" read ---7.2-. Under column heading "R.A. percent" for "11,6" read --11.2--.

Col. 17, line 6, for "contributed" read --contributes-.

ine 71, for "microna" read --microns.

Col. 18, line 27, for "the" read its-.

Col. 21, in claim 5, line 3, for "nickel" read a low alloy steel-.

Line 4, delete "about 0.05".

Col. 22, in claim 10, line 1, for "3" read -2.

Signed and sealed this 26th day of March 197A.

(SEAL) Attest EDWARD I LFLETCHEP JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents 

